The invention relates to the diffusion coating of superalloys, such as high-nickel and high-cobalt alloys, wherein a part to be treated is placed in a coating, powder pack.
It is already known that diffusion coating of metallic objects, for example nickel- and cobalt-based alloys, may be carried out by embedding the article to be coated in a powdered coating pack including:
(1) an inert filler,
(2) a vaporizable carrier ingredient, and
(3) powdered sources of metal materials to be diffused into the superalloy object.
The vaporizable carrier ingredient, usually a halide composition, acts as a flux in facilitating the initial reaction between aluminum and the alloy being treated, and also acts to accelerate the diffusion process by forming intermediate or transient compounds during the process. In general, the vaporizable carrier material has been a halide, for example a fluoride or chloride salt, such as ammonium chloride. This relatively diffusable material provides means for carrying the treating metal into the superalloy surfaces to be treated.
The metal powder, usually aluminum or choronium, is the active metal-treating agent on which the carrier material acts to facilitate diffusion of the metal into the article to be treated.
The inert filler acts primarily as a means to moderate the concentration and rate at which the carrier material and diffusing metal approach the metal article to be treated. It is also a manipulative expedient which provides the function of reducing the cost of material expended in carrying out the process. Cost of the material in a powder pack usually requires that the pack is to be used for a number of treating cycles. In such cases the pack is usually refurbished with the more-readily depleted components before proceeding from one cycle to another.
Processes of this general type have been disclosed by Wachtell and Seelig in U.S. Pat. No. 3,257,230 and by Puyear and Schley in U.S. Pat. No. 3,079,276. The object of such processes is to provide a protective outer sheathing on engineering parts subjected to high temperatures and corrosive atmospheres; for example, the turbine blades in jet aircraft engines are subjected to such temperature and environment.
Some additional prior art includes packings which utilize Co.sub.2 Al.sub.5 (e.g. PWA 273, a material sold by Pratt & Whitney Division of United Technologies, Inc. and packings such as those described in a number of patents including U.S. Pat. Nos. 3,716,398; 3,594,219; 3,577,268; 3,810,782; 4,024,294; 4,041,196; 3,979,274; and many more such patents primarily classified in U.S. Classification 117 (old) and Class 427.
There are many aspects of these diffusion processes for pack cementation coatings that it would be desirable to enhance. Some such aspects relate to the mechanical properties of the powder composition: for example, it would be very desirable for the used coating powder to be readily released from some of the very small apertures and channels that are encountered in the alloy parts being treated. Moreover, it is always desirable to improve the "hot corrosion" resistance of internal as well as external surfaces of the parts by subjecting them to treatments of the type being described.
As will be described below, the invention described herein is based on the surprising discovery of the value of certain cementatious coating powders.